1. Field of the Invention
The present invention relates to a wavelength monitor.
2. Description of the Related Art
Recently, in the field of optical communications, with the increase in speed and capacity in an optical transmission system, the Wavelength Division Multiplexing (WDM) system for performing optical multiple transmission with one optical fiber has become widespread as a core technology thereof. For performing a stable management, this WDM system needs to ensure a preparatory signal light source having the same oscillation wavelength in case of unexpected stoppage of a signal light source, which increases the maintenance cost. For suppressing this cost, a demand for an optical wavelength-variable light source capable of outputting laser light of a plurality of wavelengths by one signal light source has been increasing.
It is required for the wavelength-variable light source that the wavelength of an optical signal is stable over a long period of time, so that a light source having a function of a wavelength monitor, which monitors light emitted from a semiconductor laser, has been developed. As a representative wavelength-variable light source, a system in which the oscillation wavelength is made tunable by changing the temperature of the semiconductor laser is proposed. The tunable width of the oscillation wavelength of the semiconductor laser in this system is determined based on the operating temperature range and is only about 2 to 3 nm, so that a structure in which a plurality of semiconductor lasers are provided is often used for increasing the wavelength-variable width.
As a typical conventional technology relating to the wavelength monitor of the wavelength-variable light source, a structure is proposed in which emitted light (hereinafter may be referred to just as “exit light”) in the backward direction from a plurality of semiconductor lasers is multiplexed into one waveguide in an optical multiplexer, and the light, which is emitted from one port and is output to an optical fiber, is split by a beam splitter or the like to be partially used for the wavelength monitor (for example, Non-patent documents 1, 2, and 3, which are Japanese Patent Applications Laid-Open NO. 2002-185074, 2007-157937 and 2002-171023, respectively).
Moreover, a structure of using exit light in the backward direction from a plurality of semiconductor lasers for a wavelength monitor is proposed (for example, Non-patent documents 3 and 4, wherein document 4 is Japanese Patent Application Laid-Open No. 2003-163411).
However, there are following problems in the above conventional technologies. In the conventional method (for example, Non-patent documents 1, 2, and 3) of performing the wavelength monitoring by using the forwardly emitted exit light, a part of the light to be output to the optical fiber is split for monitoring the wavelength, so that the output power from the wavelength-variable light source decreases. Moreover, because an optical component such as the beam splitter is needed, the size of the whole wavelength-variable light source increases and accordingly the cost increases.
In the conventional method (for example, Non-patent document 4) of performing the wavelength monitoring by using exit light in the backward direction, although the above problems are solved, a point at which light is emitted from a semiconductor substrate (hereinafter may be referred to just as an “exit portion”) is different for each semiconductor laser, so that a variation in the wavelength monitoring characteristics due to an angle of incident on a filter and an incident position on the filter is a problem. For example, in Non-patent document 4 described above, exit light is guided to a position in a range in which the light to be monitored can enter by providing a backward optical waveguide, which guides the backwardly emitted exit light from a plurality of semiconductor lasers to a narrow area at the rear end of a semiconductor chip, to the semiconductor chip. However, the actual light exit positions are all different, so that a propagation angle of collimated light after transmitting through a collimator lens changes depending on the eccentricity of each light emission point seen from the collimator lens provided in the backward direction of the semiconductor chip. The frequency periodicity of the transmittance of the filter depends on the incident angle of light, so that the relationship between the light intensity of exit light from each semiconductor laser detected by a photodetector and the frequency of light becomes different, which degrades the monitoring accuracy of the wavelength of light.